Extended bandwidth semiconductor optical amplifier systems and methods

ABSTRACT

Optical devices and associated production methods are disclosed, one example of such a device takes the form of a semiconductor optical amplifier having first and second ends that at least partially define a signal propagation path. The semiconductor optical amplifier has an active layer that includes multiple quantum well stacks disposed between the first and second ends along the signal propagation path. Finally, the multiple quantum well stacks have a thickness and a width that vary along the signal propagation path.

RELATED APPLICATIONS

This application is a continuation, and claims the benefit, of U.S. Pat. application Ser. No. 10/348,641, entitled EXTENDED BANDWIDTH SEMICONDUCTOR OPTICAL AMPLIFIER, filed Jan. 21, 2003, which, in turn, claims the benefit of United States Provisional Patent Application Ser. No. 60/357,972, entitled EXTENDED BANDWIDTH SEMICONDUCTOR OPTICAL AMPLIFIER, filed Feb. 12, 2002. Both of the aforementioned applications are incorporated herein in their respective entireties by this reference.

BRIEF DESCRIPTION OF THE INVENTION

The present invention relates generally to an extended bandwidth semiconductor optical amplifier.

BACKGROUND OF THE INVENTION

In optical communication systems, optical amplifiers play an important role in compensating for losses in multiplexing, splitting, switching, or wavelength converting. Semiconductor optical amplifiers (SOA's) are used widely to meet these needs. The use of SOA's is advantageous because they are less expensive and smaller than fiber amplifiers. One problem associated with conventional SOA's, however, is that their gain spectrum is narrow, typically around thirty to forty nanometers wide. Because of their narrow gain spectrum, the usefulness of conventional SOA's is somewhat limited in WDM (Wavelength Divisional Multiplexing) applications.

Accordingly, there exists a need for an optical amplifier whose gain spectrum is wider so as to cover as much of the useful fiber-optic communications spectrum as possible.

BRIEF SUMMARY OF AN EXEMPLARY EMBODIMENT OF THE INVENTION

In general, exemplary embodiments of the invention are concerned with semiconductor optical amplifiers and associated production methods. In one exemplary embodiment, a semiconductor optical amplifier is provided that includes first and second ends that at least partially define a signal propagation path. The semiconductor optical amplifier has an active layer that includes multiple quantum well stacks disposed between the first and second ends along the signal propagation path. The multiple quantum well stacks have a thickness and a width that vary along the signal propagation path. One result of this construction is that the exemplary semiconductor optical amplifier has a relatively flatter and broader gain spectrum, as compared with semiconductor optical amplifiers that employ multiple quantum well stacks of uniform thickness.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present invention will be more readily apparent from the following description and appended claims when taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a perspective view of a semiconductor optical amplifier (SOA) according to an embodiment of the present invention;

FIG. 2 is a top view of the SOA shown in FIG. 1;

FIG. 3 is a side view of the SOA shown in FIGS. 1 and 2;

FIG. 4 is a cross-sectional view of the SOA of FIGS. 1-3 near one of its ends;

FIG. 5 is a cross-sectional view of the SOA of FIGS. 1-3 near another one of its ends;

FIG. 6 is a graph illustrating the gain spectrum of an SOA according to an embodiment of the present invention;

FIG. 7 illustrates a top view of a Semiconductor Optical Amplifier in accordance with an alternate embodiment of the present invention;

FIG. 8 illustrates a side view of a Semiconductor Optical Amplifier in accordance with an alternate embodiment of the present invention;

FIG. 9 illustrates a top view of a Semiconductor Optical Amplifier in accordance with an alternate embodiment of the present invention; and

FIG. 10 illustrates a top view of a Semiconductor Optical Amplifier in accordance with an alternate embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described. It will be appreciated that in the development of any such embodiment, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.

FIGS. 1, 2 and 3 depict a perspective view, a top view and a side view of a semiconductor optical amplifier (SOA) 50 according to an embodiment of the present invention. In FIGS. 1-3, the SOA 50 has a substrate 10 and a mesa structure 12. Masks 14 are disposed on the substrate 10 along the sides of the mesa structure 12. In the present embodiment, the substrate 10 is made of III-V semiconductor materials such as InP, and the masks 14 are made of an amorphous semiconductor material, such as amorphous silicon dioxide.

In the embodiment illustrated in FIGS. 1, 2 and 3, the masks 14 are wider at a first end 16 of the mesa structure 12 than they are at a second end 17, and the mesa structure 12 is thicker at the first end 16 than it is at the second end 17. In this embodiment, light can propagate from the first end 16 to the second end 17, or vice versa, along the length of the mesa structure 12.

With reference again to FIGS. 1, 2 and 3, the mesa structure 12 has an active layer of Multiple Quantum Well (MQW) stacks 18 that include alternating layers of barriers and quantum wells. Both the barriers and quantum wells are formed using appropriately selected III-V semiconductor materials. When properly biased, the MQW stacks 18 amplify the optical signal received at one end 16 and emit amplified optical signal at the other end. In the present embodiment, electrodes (including an electrode 21 covering the top of the mesa structure 12 and another electrode 22 covering the bottom of the substrate 10) are used to provide the proper biasing of the SOA.

The thickness of the MQW stacks 18 varies along the light propagation path of the SOA 50. The thickness of the MQW stacks 18, however, does not vary linearly from the first end 16 to the second end 17. Rather, the thickness varies in a non-linear fashion. In some portions of the SOA 50, the thickness of the MOW stacks 18 remains the same. The thicker MOW stacks preferably include thicker layers of quantum wells. According to the present invention, the gain spectrum is dependent on the thickness of the quantum wells. Therefore, at the thicker end 16, the gain is peaked at longer wavelengths due to the thicker MQW stacks. At the thinner end 17, the gain is peaked at shorter wavelengths. One may consider the MQW stacks 18 to be analogous to many SOA's of small gain connected in series, each SOA having a slightly different gain peak. Optical signals with a flat and broad gain spectrum, which is shown in FIG. 6 as an example, can thus be produced. The gain spectrum shown in FIG. 6 is wider than that of conventional semiconductor optical amplifiers.

Attention now turns to FIGS. 4-5 and a fabrication processes for fabricating the SOA 50. FIG. 4 illustrates the cross section of SOA 50 across line A-A, and FIG. 5 illustrates the cross section across line B-B of FIG. 3. This fabrication process utilizes a technique known as Selective Area Growth (SAG), which is a technique that allows the growth rate to be varied as a function of position due to the proximity of a mask. The SAG technique involves, in the present embodiment, forming masks 14 over the semiconductor substrate 10 so as to permit vapor phase growth of semiconductor crystals in unmasked area. The masks 14 are preferably formed by depositing an amorphous material not conducive to epitaxial growth of semiconductor materials on the substrate 10 and patterning the amorphous material to expose the unmasked area. The width of the gap between the masks 14 varies along the path in which light propagates. In this particular embodiment, the gap between the masks 14 is smaller at the first end 16 than the gap at the second end 17. As a result, the growth rate is higher at the first end 16. In other embodiments, the gap between the masks is larger at the light-receiving end than at the light-emitting end.

With reference still to FIGS. 4-5, a Metal-Organic Chemical Vapor Deposition (MOCVD) process is carried out to deposit semiconductor materials, including semiconductor materials for forming the MQW stacks 18, on the unmasked area. The thickness of the semiconductor materials grown on the exposed area varies according to the width of the exposed area. In particularly, where the gap between the masks 14 is small, the growth rate is higher, resulting in a thicker layer of semiconductor materials. Conversely, where the gap between the masks 14 is large, a thinner layer of semiconductor materials will be deposited. A product of the MOCVD process is the mesa structure 12 where the MQW stacks 18 are thicker near the first end 16 and near at the second end 17.

In some embodiments, although the MQW stacks 18 are thicker at one end than at another, the process can be controlled such that the cross-sectional area of the MQW stacks perpendicular to the propagation direction of optical signals remains substantially constant. Furthermore, in the embodiment shown in FIGS. 4-5, the mesa structure 12 has a rectangular cross-sectional area. In other embodiments, the mesa structure has a trapezoidal cross-sectional area. In yet other embodiments, the mesa structure has an inverted trapezoidal cross-sectional area.

Other processes, such as the formation of electrodes and application of anti-reflective coatings to the first end 16 and the second end 17, are carried out so as to complete the process of manufacturing the SOA 50.

FIGS. 7 and 8 illustrate a top view and a side view, respectively, of a Semiconductor Optical Amplifier 70 in accordance with an alternate embodiment of the present invention. The SOA 70 is formed by cascading two SOA's 50. In the embodiment shown in FIGS. 7 and 8, the thickness of the MQW stacks 18 varies along the direction of light propagation from one end to the other. As shown, a middle portion of the MQW stacks 18 is thicker than the end portions. The thicker MOW stacks 18 preferably include thicker layers of quantum wells. This embodiment of the present invention also has a flat and broad gain spectrum.

FIGS. 9 and 10 illustrate a top view and a side view, respectively, of a Semiconductor Optical Amplifier 80 in accordance with an alternate embodiment of the present invention. The SOA 80 is formed by cascading two SOA's 50. In the embodiment shown in FIGS. 9 and 10, the thickness of the MOW stacks 18 varies along the direction of light propagation from one end to the other. Particularly, the end portions of the MQW stacks 18 is thicker than the middle portion. The thicker MQW stacks 18 preferably include thicker layers of quantum wells. Similar to the embodiments described above, this embodiment of the present invention also has a flat and broad gain spectrum.

While the present invention has been described with reference to a few specific embodiments, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications may occur to those skilled in the art having the benefit of this disclosure without departing from the inventive concepts described herein. For instance, multiple SOA's of the present invention can be cascaded with SOA's with substantially flat MOW stacks to form SOA's with a broad gain spectrum. The present invention can be used in many different types of apparatuses as well. For example, SOA's of the present invention can be integrated on a chip with other active components to either boost output in the case of lasers or electroabsorption lasers. SOA's of the present invention can also be used as optical amplifiers at the input stage of an optical receiver. 

1. A semiconductor optical amplifier, comprising: first and second ends that lie along a signal propagation path; and an active layer that includes multiple quantum well stacks disposed between the first and second ends and comprising at least a portion of the signal propagation path, the multiple quantum well stacks having both a thickness and a width, and the multiple quantum well stacks being constructed such that both the thickness and the width vary along the signal propagation path.
 2. The semiconductor optical amplifier as recited in claim 1, wherein the thickness of the multiple quantum well stacks varies non-linearly along the signal propagation path.
 3. The semiconductor optical amplifier as recited in claim 1, wherein at least a portion of the multiple quantum well stacks has a thickness that is substantially constant.
 4. The semiconductor optical amplifier as recited in claim 1, wherein the thickness of the multiple quantum well stacks is relatively greater at a relatively narrower portion of the multiple quantum well stacks than at a relatively wider portion of the quantum well stacks.
 5. The semiconductor optical amplifier as recited in claim 1, wherein a cross-sectional area of the multiple quantum well stacks oriented in a direction substantially perpendicular to the signal propagation path is substantially constant.
 6. The semiconductor optical amplifier as recited in claim 1, wherein the multiple quantum well stacks comprise at least a portion of a mesa structure.
 7. The semiconductor optical amplifier as recited in claim 6, wherein the mesa structure has a cross-section substantially in the form of one of: a rectangle; a trapezoid; and, an inverted trapezoid.
 8. The semiconductor optical amplifier as recited in claim 1, wherein the multiple quantum well stacks comprise alternating layers of barriers and quantum wells.
 9. The semiconductor optical amplifier as recited in claim 8, wherein a thickness of the quantum wells varies along the signal propagation path.
 10. The semiconductor optical amplifier as recited in claim 1, wherein the semiconductor optical amplifier is configured to permit light to travel in either direction along the signal propagation path.
 11. The semiconductor optical amplifier as recited in claim 1, further comprising: a substrate having upper and lower surfaces, the upper surface being located proximate the active layer; a first electrode positioned proximate the lower surface of the substrate; and a second electrode positioned above the active layer.
 12. An optical device, comprising: a first semiconductor optical amplifier; and a second semiconductor optical amplifier that cooperates with the first semiconductor optical amplifier to define a signal propagation path that has a middle portion and two end portions, the middle portion having a thickness different from that of either of the end portions, and each of the first and second semiconductor optical amplifiers including an active layer, the respective active layer of each semiconductor optical amplifier including multiple quantum well stacks that form a portion of the signal propagation path, the multiple quantum well stacks of each active layer having both a thickness and a width, and the multiple quantum well stacks of each active layer being constructed such that both the thickness and the width vary along the signal propagation path.
 13. The optical device as recited in claim 12, wherein the middle portion of the signal propagation path defined by the first and second semiconductor optical amplifiers is relatively thicker than the end portions.
 14. The optical device as recited in claim 12, wherein the middle portion of the signal propagation path defined by the first and second semiconductor optical amplifiers is relatively thinner than the end portions.
 15. The optical device as recited in claim 12, wherein the thickness of the multiple quantum well stacks of at least one of the active layers varies non-linearly along the signal propagation path.
 16. The optical device as recited in claim 12, wherein at least a portion of the multiple quantum well stacks of at least one of the active layers has a thickness that is substantially constant.
 17. The optical device as recited in claim 12, wherein the optical device is configured to permit light to travel in either direction along the signal propagation path.
 18. The optical device as recited in claim 12, wherein the multiple quantum well stacks of each of the active layers comprise alternating layers of barriers and quantum wells.
 19. The optical device as recited in claim 18, wherein a thickness of the quantum wells varies along the signal propagation path.
 20. The optical device as recited in claim 12, wherein the first and second semiconductor optical amplifiers abut each other.
 21. The optical device as recited in claim 12, wherein, for each active layer, the thickness of the multiple quantum well stacks is relatively greater at a relatively narrower portion of the multiple quantum well stacks than at a relatively wider portion of the quantum well stacks.
 22. An optical device, comprising: a laser; and semiconductor optical amplifier configured and arranged to boost a laser output, the semiconductor optical amplifier comprising: first and second ends that lie along a signal propagation path; and an active layer that includes multiple quantum well stacks disposed between the first and second ends and comprising at least a portion of the signal propagation path, the multiple quantum well stacks having both a thickness and a width, and the multiple quantum well stacks being constructed such that both the thickness and the width vary along the signal propagation path.
 23. The optical device as recited in claim 22, wherein the laser comprises an electroabsorption laser.
 24. The optical device as recited in claim 22, wherein the thickness of the multiple quantum well stacks is relatively greater at a relatively narrower portion of the multiple quantum well stacks than at a relatively wider portion of the quantum well stacks.
 25. The optical device as recited in claim 22, wherein the thickness of the multiple quantum well stacks varies non-linearly along the signal propagation path.
 26. The optical device as recited in claim 22, wherein at least a portion of the multiple quantum well stacks has a thickness that is substantially constant.
 27. The optical device as recited in claim 22, wherein a cross-sectional area of the multiple quantum well stacks oriented in a direction substantially perpendicular to the signal propagation path is substantially constant.
 28. The optical device as recited in claim 22, wherein the multiple quantum well stacks comprise at least a portion of a mesa structure.
 29. The optical device as recited in claim 22, wherein the semiconductor optical amplifier is configured to permit light to travel in either direction along the signal propagation path.
 30. An optical device, comprising: an optical receiver having an input stage; and a semiconductor optical amplifier configured and arranged for communication with the input stage of the optical receiver, the semiconductor optical amplifier comprising: first and second ends that at least partially define a signal propagation path; and an active layer that includes multiple quantum well stacks disposed between the first and second ends and comprising at least a portion of the signal propagation path, the multiple quantum well stacks having both a thickness and a width, and the multiple quantum well stacks being constructed such that both the thickness and the width vary along the signal propagation path.
 31. The optical device as recited in claim 30, wherein the thickness of the multiple quantum well stacks is relatively greater at a relatively narrower portion of the multiple quantum well stacks than at a relatively wider portion of the quantum well stacks.
 32. The optical device as recited in claim 30, wherein the thickness of the multiple quantum well stacks varies non-linearly along the signal propagation path.
 33. The optical device as recited in claim 30, wherein at least a portion of the multiple quantum well stacks has a thickness that is substantially constant.
 34. The optical device as recited in claim 30, wherein a cross-sectional area of the multiple quantum well stacks oriented in a direction substantially perpendicular to the signal propagation path is substantially constant.
 35. The optical device as recited in claim 30, wherein the multiple quantum well stacks comprise at least a portion of a mesa structure.
 36. The optical device as recited in claim 30, wherein the semiconductor optical amplifier is configured to permit light to travel in either direction along the signal propagation path.
 37. An optical device, comprising: a first semiconductor optical amplifier; and a second semiconductor optical amplifier that cooperates with the first semiconductor optical amplifier to define a signal propagation path of varying thickness, and each of the first and second semiconductor optical amplifiers including an active layer, the respective active layer of each semiconductor optical amplifier including multiple quantum well stacks that form a portion of the signal propagation path, the multiple quantum well stacks of the active layer of the first semiconductor optical amplifier having a variable width and relatively constant thickness along the signal propagation path, and the multiple quantum well stacks of the active layer of the second semiconductor optical amplifier having a width and thickness that vary along the signal propagation path.
 38. The optical device as recited in claim 37, wherein the first and second semiconductor optical amplifiers abut each other.
 39. The optical device as recited in claim 37, wherein, for the active layer of the second semiconductor optical amplifier, the thickness of the multiple quantum well stacks is relatively greater at a relatively narrower portion of the multiple quantum well stacks than at a relatively wider portion of the quantum well stacks.
 40. A method for forming a semiconductor optical amplifier, the method comprising: masking a substrate so as to define an unmasked gap having a parameter whose value varies; and growing semiconductor materials on the unmasked gap so that an active layer including at least one multiple quantum well stack is formed, the growing of the semiconductor materials being performed such that a growth rate of the semiconductor materials in the unmasked gap varies in correspondence with variations in the value of the gap parameter.
 41. The method as recited in claim 40, wherein the gap parameter comprises a width of the gap.
 42. The method as recited in claim 41, wherein the growth rate of the semiconductor materials is relatively higher in a relatively narrower portion of the unmasked gap than in a relatively wider portion of the unmasked gap.
 43. The method as recited in claim 40, wherein an MOCVD process is used to deposit the semiconductor materials on the unmasked gap.
 44. The method as recited in claim 40, wherein masking the substrate comprises: depositing, on the substrate, an amorphous material nonconductive to epitaxial growth of semiconductor materials; and patterning the amorphous materials to expose the unmasked gap. 